Compact reformers find increasing use for fuel cells and small-scale hydrogen generators such as on-board reformers. In addition to fuel cell vehicle application, on-board reformers can be used to supply small amounts of hydrogen or syngas (mixture of hydrogen and carbon monoxide) either to gasoline internal combustion engines or to pollution treatment devices of the vehicle to increase the NOx removal efficiency. The three most important requirements of on-board reformers are compactness, fast start-up capability and low pressure-drop characteristics. Catalytic partial oxidation (CPOX) reformers can meet these requirements.
Numerous documents related to CPOX processes are contained in the literature. However, the majority of these processes are restricted to catalyst compositions and catalyst structures, which need further steps of development for the complete form of the reactor assembly to be readily applied in the real practices including the start-up device.
A. T. Ashcroft et al. (“Selective oxidation of methane to synthesis gas using transition metal catalysts”, Nature, Vol. 344. 319-321, 1990) disclose methane oxidation to syngas on several rare earth-ruthenium mixed oxide catalysts. At contact times of about 0.1 s and temperatures above 750° C., the powdered catalysts effectively convert methane to syngas.
Similar example of the mixed oxide catalyst utilized in the CPOX reaction disclosed in the U.S. Pat. No. 5,447,705 is the perovskite of LnxAl-yByO3 structure wherein Ln is at least on of La, Ce, Pr, Nd, Sm, En, Dy, Ho or Ef, A is Ti, Cr, Fe, Ru, Co, Rh, or Ni, B is Ti, Cr, Fe, Ru, Co, Rh or Ni.
Another example of the CPOX catalyst composition by P. D. F. Vernon et al. (“Partial oxidation of methane to synthesis gas, and carbon dioxide as an oxidizing agent for methane conversion”, Catalysis Today, Vol. 13, 417-426, 1992) discloses that the transition metals such as nickel, ruthenium, rhodium, palladium, platinum and iridium supported on inert oxides can catalyze methane to synthesis gas to the equilibrium value.
Similar disclosure of CPOX catalyst composition is described in U.S. Pat. No. 5,720,901, wherein rhodium, ruthenium and iridium were used to process sulfur-containing light hydrocarbons.
In terms of designing the CPOX reactor, there are problems associated with the shape of the catalyst. The powder form catalyst can not tolerate the feedstock gas at high gas space velocities due to severe pressure drop. The pellet catalyst can alleviate the pressure drop problems to a certain extent but less than optimum heat transfer can facilitate hot spots, resulting in performance degradation of catalysts. An open channel structured monolith catalyst can have more uniform temperatures through the heat dissipation by connecting walls while minimizing the pressure drop. Thus, the open channel structured monolith catalyst is the preferred form of the catalyst that can be adopted in the CPOX reactor operated at high space velocities.
The example of monolith catalyst application in CPOX is found in U.S. Pat. No. 5,648,582 disclosing that the ceramic monolith washcoated with a supported metal catalyst converted methane at space velocities within the range of 800,000 hr-1 to 12,000,000 hr-1.
When the catalyst in the form of ceramic monolith is used at high temperatures, the problem of catalyst breakdown can arise because of the susceptibility of the refractory monolithic catalytic structure to thermal shock. U.S. Pat. No. 5,639,401 discloses that the rhodium catalyst on alumina foam that had been subjected to the CPOX reaction became shattered into many fragments. In the same patent, the use of the ZrO2-based monolith type CPOX catalysts is disclosed that are resistant to thermal shock.
U.S. Pat. No. 5,786,296 discloses thin-walled monolithic iron oxide structures made from steels for the purpose of using them for heat exchangers, mufflers and catalyst carriers. Compared to ceramic monoliths, the monolith made of thin metal plates is more resistant to the thermal shock. Other advantages of thin-walled metal monolith are rapid heat-up capability and more uniform temperature distribution due to the higher thermal conductivity and the smaller thermal mass. Thus, metal monolith-based catalysts are well suited for compact CPOX reformers that require fast start-up capability. Although the possibility of utilizing metal monolith for the CPOX process is claimed in U.S. Pat. No. 5,648,582 and U.S. Pat. No. 6,221,280 B1, examples of metal monolith-based CPOX catalysts are not disclosed.
Another unique feature of the metal monolith is that it can be used as a start-up device for the CPOX reaction. When electricity is supplied to the metal monolith, it became heated resistively to raise the temperature of the gas passing through the channels of the monolith. The device is called “electrically heated (heatable) catalyst (EHC)”. T. Kirchner et al. (“Optimization of the cold-start behavior of automotive catalysts using an electrically heated pre-catalyst”, Chemical Engineering Science, Vol. 51, 2409-2418, 1996) and S. R. Nakouzi et al. (“Novel concept prototype low-power-consumption electrically heatable catalysts”, AIChE Journal, Vol. 44, 184-187, 1998) disclose the use of electrically heated catalyst as the device to light-off combustion of CO and hydrocarbons in automotive exhaust gases. Since the use of EHC does not include any flame during operation, it is a safe start-up device. No EHC has been disclosed in the literature as a start-up device for the CPOX reaction.
Other method of igniting the CPOX reaction disclosed in U.S. Pat. No. 6,221,280 B1 is the use of an external heater to raise the temperature of the reactant gas to above the light-off temperature. The CPOX reactor assemblage employing an external heater is neither compact nor fast to be started. Another method of igniting the CPOX reaction disclosed in U.S. Pat. No. 5,648,582 is a sparker-ignited complete combustion of NH3 or CH4 to heat the catalyst to about 1000° C. followed by feeding the reactant gas mixture at a stoichiometric ratio of CH4/O2. The use of spark-ignited complete combustion as the start-up method may pose a danger of explosion and destruction of catalysts.
In addition to the adoption of very active catalyst, preheating the reactant gas mixture can also make the performance enhancement of the CPOX process. U.S. Pat. No. 5,648,582 discloses that air heated at 460° C. resulted in the significantly higher methane conversion than air at 25° C., however, the method of air preheating is not disclosed.